C++ Program For Making Middle Node Head In A Linked List

Given a singly linked list, find middle of the linked list and set middle node of the linked list at beginning of the linked list. 
Examples:

Input: 1 2 3 4 5 
Output: 3 1 2 4 5

Input: 1 2 3 4 5 6
Output: 4 1 2 3 5 6 

The idea is to first find middle of a linked list using two pointers, first one moves one at a time and second one moves two at a time. When second pointer reaches end, first reaches middle. We also keep track of previous of first pointer so that we can remove middle node from its current position and can make it head.

C++

// C++ program to make middle node
// as head of linked list.
#include <bits/stdc++.h>
using namespace std;

// Link list node
class Node
{
    public:
    int data;
    Node* next;
};

/* Function to get the middle and set at
   beginning of the linked list*/
void setMiddleHead(Node** head)
{
    if (*head == NULL)
        return;

    // To traverse list nodes one by one
    Node* one_node = (*head);

    // To traverse list nodes by skipping
    // one.
    Node* two_node = (*head);

    // To keep track of previous of middle
    Node* prev = NULL;
    while (two_node != NULL && two_node->next != NULL)
    {

        // For previous node of middle node
        prev = one_node;

        // Move one node each time
        two_node = two_node->next->next;

        // Move two node each time
        one_node = one_node->next;
    }

    // Set middle node at head
    prev->next = prev->next->next;
    one_node->next = (*head);
    (*head) = one_node;
}

// To insert a node at the beginning of
// linked list.
void push(Node** head_ref, int new_data)
{
    // Allocate node
    Node* new_node = new Node();

    new_node->data = new_data;

    // Link the old list of the new node
    new_node->next = (*head_ref);

    // Move the head to point to the new node
    (*head_ref) = new_node;
}

// A function to print a given linked list
void printList(Node* ptr)
{
    while (ptr != NULL)
    {
        cout << ptr->data << " ";
        ptr = ptr->next;
    }
    cout<<endl;
}

// Driver code
int main()
{
    // Create a list of 5 nodes
    Node* head = NULL;
    int i;
    for (i = 5; i > 0; i--)
        push(&head, i);

    cout << " list before: ";
    printList(head);

    setMiddleHead(&head);

    cout << " list After: ";
    printList(head);

    return 0;
}
// This is code is contributed by rathbhupendra

Output:

list before: 1 2 3 4 5
list After : 3 1 2 4 5 

Time Complexity: O(n) where n is the total number of nodes in the linked list.

Auxiliary Space: O(1)

Please refer complete article on Make middle node head in a linked list for more details!